CompatHelper: bump compat for "Plots" to "0.28"

Author: github-actions[bot]

Project.toml

@@ -20,12 +20,12 @@ SparseArrays = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [compat]
-julia = "1.0"
 Colors = "0.9.2"
 DSP = "0.6.1"
 DelayDiffEq = "5.6"
 IterTools = "1.0"
 LaTeXStrings = "1.0"
 OrdinaryDiffEq = "5.2"
-Plots = "0.24, 0.25, 0.26, 0.27"
+Plots = "0.24, 0.25, 0.26, 0.27, 0.28"
 Polynomials = "0.6.0"
+julia = "1.0"